1. Field of This Invention
This invention relates to a process for obtaining maleic acid anhydride from the aqueous maleic acid solutions obtained from the gas phase oxidation of organic compounds having at least four carbon atoms by washing out the reaction gases with water.
2. Prior Art
Maleic acid anhydride is produced industrially by gas phase oxidation of benzene, butane, butenes, butadiene, etc., in the presence of suitable catalysts and with a large excess of air as oxidation agent. The maleic acid anhydride formed thereby may be separated subsequently in the solid or liquid state in suitable separators. Corresponding to the low steam pressure of maleic acid anhydride in the reaction gas and to the condition of condensing above the thawing point of water (ice), part to the maleic acid anhydride formed passes in gas form through the separator and must be washed out of the gas stream with water or suitable absorption agents. In the case of the use of water as an absorption agent, more or less concentrated aqueous maleic acid solutions with a content of about 20 to 60 percent of maleic acid result.
In the industrial processes which are operable or commercially usable nowadays, no less than 50 percent of the resultant maleic acid anhydride is obtained from an aqueous solution in the form of maleic acid, for example, by means of a film evaporator. Such maleic acid has to be converted in yet another step in the industrial process into maleic acid anhydride.
According to some patents, maleic acid anhydride is absorbed in an organic solvent, whereby the conversion of maleic acid anhydride into the acid and the reconversion of this acid into maleic acid anhydride can be avoided. But the fact alone that none of such patents have ever obtained any commercial value proves to the art that the method of obtaining maleic acid in water is still the most suitable one.
The obtention of maleic acid anhydride from its aqueous solutions can be achieved by several methods, which generally can be classified in two main groups.
The most widely used and customary method consists of the removal of water by azeotropic distillation under atmospheric pressure in the presence of a suitable organic solvent, such as, o-xylene, pseudocumene, etc. At the same time, not only is the solution water removed, maleic acid is converted into maleic acid anhydride by splitting off one mole of water. Subsequent distillation will remove the organic solvent and the thusly-obtained crude maleic acid anhydride is subjected to further purification.
A relatively new method of operation consists of the removal of the water by vacuum distillation. In this way the maleic acid is converted into maleic acid anhydride at temperatures of 150.degree. to 160.degree. C. and the solvent used for this is maleic acid anhydride itself. The conversion of maleic acid into maleic acid anhydride therefore always takes place by thermal treatment in the presence of a solvent which may be o-xylene or maleic acid anhydride.
The main disadvantage lies in the fact, that because of the high temperatures needed for the thermal treatment, a considerable portion of the maleic acid isomerizes to fumaric acid. The fumaric acid which is insoluble under reaction conditions, has a high melting point (about 386.degree. C.) and consequently causes deposits in operating installations, which represents a loss in yield. Also, such installations periodically must be shut down and cleaned, which also raises ecological problems.